Surprisingly little is known about how antigens (Ags) enter the immune system and induce B cells to produce sustained levels of neutralizing antibodies (Abs), which protect us against deadly viruses and bacteria. The major goal of this proposal is to elucidate processes required for the development of long-lived memory B cells, which are initiated after Ags are taken up by splenic dendritic cell (DC) subsets. We will define the mechanisms by which Ag targeting to marginal zone (MZ) DC and plasmacytoid DC (pDC) subsets induce the development of memory B cells and humoral immunity. Our Aims are: 1. To define how to regulate CD4 and CD8 T cell and humoral immune responses by targeting Ags to plasmacytoid DCs in vivo using Ags coupled to monoclonal antibodies (mAbs) specific for the human CLR, BDCA2 and transgenic (Tg) mice expressing BDCA2 only on pDCs. 2. To define if and how BDCA2 signaling inhibits type I IFN production by pDCs in vivo and whether this inhibition can alter the course of a lupus-like autoimmune disease. And 3. To define how extrafollicular Ab responses are generated by targeting Ags to MZ DCs in vivo and define what signals shift extrafollicular Ab responses induced via MZ DC targeting into an immune response leading to GC formation and long-lived, high-affinity Abs. We will investigate the role of MHC class II and CD22 in MZ DC-based Ag targeting and characterize the molecular processes required for MZ DCs to activate extrafollicular TEFH cells. These studies may lead to new insights into how to induce and regulate immunologic memory, and in particular humoral immunity. They may also help advance the field of B cell biology by helping to define the in vivo pathways leading to somatic mutation in B cells and affinity maturation. The proposed studies also may lead to new Ag targeting technology useful for the creation of effective vaccines which induce strong neutralizing antibody responses against important pathogens like H5N1 pandemic FLU, HIV, and hepatitis C viruses.